The invention relates to a scanning microscope for observing an object placed in an object plane. An objective system focuses radiation emitted by a coherent radiation source to a radiation spot in the object plane. Two radiation detectors are arranged so that discrete halves of the radiation beam from the radiation spot are detected by respective detectors. The spot is preferably diffraction-limited. A microscope of this type is eminently suitable for observing phase structures in an object such as a biological cell or a piece of semiconductor material which has undergone one or more process steps for providing an integrated circuit therein.
A microscope of this type is described in the Article "Improved imaging of phase gradients in scanning optical microscopy" by D. K. Hamilton et al. in the magazine "Journal of Microscopy", Vol. 135, Pt. 3, pages 275-286. In the known microscope the radiation passing through the object is detected by two radiation-sensitive detectors each receiving approximately half of the radiation cone and converting it into an electrical signal. The detectors are juxtaposed and may be partly covered by a diaphragm so that the shape of the radiation-sensitive surface of the detectors is influenced. A high resolution can be attained by suitably choosing this shape, that is to say, by adapting it to the nature of the object to be observed.
However, if the object is not well focussed or if it has a finite thickness, the radiation detected by the detectors is greatly influenced by structures outside the image plane of the objective system. For example, radiation from an amplitude structure present outside the image plane may influence the signal which originates from a phase structure in the image plane. The amplitude or phase signal derived from the detector signals is then no longer an exact reproduction of a structure in the object plane. Moreover, there is of course a loss of definition in the observed image.